Laval J
Groupe de Réparation des lésions radio et chimio-induites, URA 147 CNRS, Institut Gustave-Roussy, VILLEJUIF, France.
Pathol Biol (Paris). 1996 Jan;44(1):14-24.
Oxidative stress occurs in cells when the equilibrium between prooxidant and antioxidant species is broken in favor of the prooxidant state. It is due to reactive oxygen species (ROS) generated either by the cellular metabolism such as phagocytosis, mitochondrial respiration, xenobiotic detoxification, or by exogenous factors such as ionizing radiation or chemical compounds performing red-ox reactions. Some ROS are extremely reactive and interact with all the macromolecules including lipids, nucleic acids and proteins. Cells have numerous defence systems to counteract the deleterious effects of ROS. Proteins and small molecules specifically eliminate ROS when they are formed. There are three species of superoxyde dismutases which transform the superoxyde anion O2- in hydrogen peroxyde H2O2 which in turn will be destroyed by peroxysomal catalase or by various peroxydases. There are numerous small molecules in the cell such as glutathion, alpha-tocopherol, vitamines A and C, melanine, etc. which are antioxydant molecules. ROS escaping destruction generate various lesions in DNA such as base modifications, degradation products of deoxyribose, chain breaks. These various lesions have been characterized and it is possible to quantitate them in the DNA of cells which have been irradiated or treated by free radical generating systems. The biological properties of the bases modified by ROS have been established. For example C8-hydroxyguanine (8-oxoG) is promutagenic since, if present in DNA during replication, it leads to incorporation of dAMP residues, leading to transversion mutation (GC-->TA). Purines whose imidazole ring is opened (Fapy residues) are stops for the DNA polymerase during DNA replication and are therefore potentially lethal lesions for the cell. Oxidized pyrimidines have comparable coding properties. Efficient DNA repair mechanisms remove these oxidized bases. In Escherichia coli cells, endonuclease III (NTH protein) and endonuclease VIII (NEI protein) excise many oxidized pyrimidines, whereas the FPG protein (formamidopyrimidine-DNA-glycosylase) eliminates 8-oxoG and Fapy lesions. Besides its DNA glycosylase activity, the protein FPG has a beta-lyase activity incising DNA at abasic site by a beta-delta elimination mechanism, and a dRPase activity. The FPG protein has a zinc finger motive which is mandatory for the recognition of its substrate. Mammalian cells have similar DNA repair proteins and it should be emphazized that there is conservation of the different functions and in most cases a remarquable homology of the amino acids sequences from E. coli to man.
当促氧化剂和抗氧化剂之间的平衡被打破，细胞内就会发生氧化应激，且这种平衡向促氧化剂状态倾斜。这是由于细胞代谢（如吞噬作用、线粒体呼吸、外源性物质解毒）产生的活性氧（ROS），或是由外源性因素（如电离辐射或进行氧化还原反应的化合物）导致的。一些ROS具有极高的反应活性，能够与包括脂质、核酸和蛋白质在内的所有大分子相互作用。细胞拥有众多防御系统来对抗ROS的有害影响。当ROS形成时，蛋白质和小分子会特异性地清除它们。有三种超氧化物歧化酶能将超氧阴离子O₂⁻转化为过氧化氢H₂O₂，而后者又会被过氧化物酶体过氧化氢酶或各种过氧化物酶破坏。细胞内有许多小分子，如谷胱甘肽、α-生育酚、维生素A和C、黑色素等，它们都是抗氧化分子。未被破坏的ROS会在DNA中产生各种损伤，如碱基修饰、脱氧核糖降解产物、链断裂。这些不同的损伤已得到表征，并且可以在受到辐射或自由基产生系统处理的细胞DNA中对其进行定量。ROS修饰的碱基的生物学特性也已确定。例如，C8-羟基鸟嘌呤（8-氧代鸟嘌呤，8-oxoG）具有促突变性，因为在复制过程中如果它存在于DNA中，会导致dAMP残基的掺入，从而导致颠换突变（GC→TA）。咪唑环打开的嘌呤（Fapy残基）在DNA复制过程中会使DNA聚合酶停止，因此对细胞来说可能是致命损伤。氧化嘧啶具有类似的编码特性。高效的DNA修复机制会去除这些氧化碱基。在大肠杆菌细胞中，核酸内切酶III（NTH蛋白）和核酸内切酶VIII（NEI蛋白）会切除许多氧化嘧啶，而FPG蛋白（甲酰胺嘧啶-DNA-糖基化酶）会消除8-oxoG和Fapy损伤。除了其DNA糖基化酶活性外，FPG蛋白还具有β-裂合酶活性，可通过β-δ消除机制在无碱基位点切割DNA，以及具有dRPase活性。FPG蛋白具有一个锌指基序，这对于识别其底物是必不可少的。哺乳动物细胞具有类似的DNA修复蛋白，应该强调的是，不同功能得以保留，并且在大多数情况下，从大肠杆菌到人类，氨基酸序列具有显著的同源性。
